1. Field of the Invention
The present invention relates to a phase-change type optical recording medium, and more particularly to a phase-change type optical recording medium which permits high-speed direct overwriting.
2. Description of the Related Art
As a practical example of an optical disc made of a phase-change recording material and permitting overwriting, a so-called DVD-RAM has been marketed which realizes a linear velocity of 6 m/second, a bit length of 0.41 xcexcm/second, a track pitch of 0.74 xcexcm, a laser wavelength of about 650 nm, a data transfer rate of 11 Mbps and a recording capacity of 2.6 GB.
To realize a larger capacity and a higher transfer rate, it is effective to reduce the spot size of a recording laser beam to raise the linear recording velocity. As a specific method of reducing the spot size of the recording laser beam, a method of shortening the wavelength of the laser beam and a method of enlarging the numerical aperture of the object lens are exemplified.
When both the method of shortening the wavelength of the laser beam and the method of enlarging the numerical aperture are employed, the spot size can be further reduced as compared with a structure in which only one of these methods is employed. When a violet laser beam having a wavelength of about 400 nm is employed as the light source and an objective lens having a numerical aperture (NA) of 0.85 is employed, higher density recording is theoretically permitted.
On the other hand, conditions under which high speed direct overwriting is permitted for the phase-change optical disc become more severe for the following reasons.
In general, the phase-change optical disc is irradiated with a large-power laser beam so that the temperature of the recording layer is raised to a level not lower than the melting point of the recording layer. The recording layer is melted, and then the temperature is rapidly lowered, resulting in writing being performed. The recorded marks are crystallized, that is, erased because the recording layer is maintained in a temperature range between a temperature at which the crystallization of the recording layer is started and the melting point for a time required for the recording layer to be formed into crystal.
When the foregoing method of shortening the wavelength of the laser beam and the method of enlarging the numerical aperture of the objective lens are employed, the temperature of a position on the optical disc is changed in a time shorter than the time required for the conventional structure under a condition that the linear recording velocity is high. FIG. 1 shows a result of calculations of a process of change of the temperature at one point on an optical disc as time elapses. As can be understood from FIG. 1, as the linear velocity is raised, and as the numerical aperture NA of the objective lens is enlarged, the time for maintaining the temperature not lower than the crystallizing temperature (for example, assuming that the temperature is 400xc2x0 C.) is maintained is shortened.
Therefore, a recording material having the crystallizing velocity which is the same as that of the conventional material encounters difficulty in crystallizing amorphous marks, that is, in erasing the amorphous marks.
So-called direct overwriting (DOW) is considered with which the power level of one laser beam is controlled in terms of time to perform recording. If the spot size is too small or if the linear recording velocity is too high, distortion of the shapes of marks is increased due to the difference in the physical properties between amorphous and crystal which is a peculiar problem for phase-change recording. That is, when a next mark is overwritten on a written recording mark, the size of a recording mark is, under the foregoing condition, enlarged as compared to a case in which the next mark is newly written on a crystal portion in which no data has been written.
The foregoing fact is caused from the following differences from the crystal phase: the response (an optical constant) of the amorphous phase with respect to a laser beam; a process of conduction of heat (heat conductivity) generated due to reactions with the laser beam; and a method of using generated heat (latent heat is not required when dissolving is performed). When the spot size is large and the linear velocity is low, change in the temperature of the recording film is moderate in terms of time. Heat is conducted prior to movement of the existing mark to the laser beam, maintaining the mark at the previous crystallizing temperature. Therefore, a state similar to a state in which no mark exists can be realized (previous crystallization) and the foregoing problem does not arise.
The foregoing problems virtually inhibit a usual phase-change recording disc having a four-layered structure consisting of a ZnSxe2x80x94SiO2 layer, a recording layer, a ZnSxe2x80x94SiO2 layer and a reflecting layer, from having a higher density and a higher transfer rate. For example, deterioration in the value of jitters as the linear velocity is raised as shown in FIG. 2 has been confirmed as a result of experiments.
As a way of overcoming the foregoing problem, it might be considered feasible to raise the crystallizing velocity of the recording layer. That is, time required to complete crystallization could be shortened so as to raise the erasing ratio and facilitate previous crystallization.
However, a material, the phase of which is reversibly changed, which has a crystallizing velocity higher than those of the conventional materials and which can be applied to an optical disc, cannot virtually be obtained.
As an alternative to raising the crystallizing velocity of the recording material, techniques for raising the crystallizing velocity of the recording layer by providing a crystallization enhancing material, which is effective to raise the crystallizing velocity of amorphous, such that the material is in contact with the recording layer have been disclosed. The foregoing techniques have been disclosed in Japanese Patent Laid-Open No. 1-92937, Japanese Patent Laid-Open No. 6-195747 and Japanese Patent Application No. 9-532424.
The foregoing methods, however, encounter a problem in that the preservation stability of the recording marks deteriorates in a temperature range under a condition of daily use. Another problem arises in that the recording mark is undesirably erased even with a laser beam for reproducing data.
As a method structured individually from the foregoing enhancement of crystallization, methods of a type inverting the absorption ratio of amorphous which is usually higher than that of crystal is inverted (the temperature raising velocity of the crystal portion and that of the amorphous portion are balanced) by controlling the lamination structure of the thin optical film have been disclosed. The foregoing method is a so-called absorption-ratio control method which has been disclosed in Japanese Patent Laid-Open No. 8-124218 and Japanese Patent Laid-Open No. 9-91755.
The foregoing method, however, suffers from a problem in that the degree of freedom of optical design (absorptance, reflectance and so forth) decreases. Another problem arises in that satisfactory durability against repeated overwriting cannot be realized.
An even greater problem is that conditions in which the diameter of the spot is reduced and the linear velocity is high encounter unsatisfactory basic erasing performance (for example, erasing ratio obtained with DC light) which is required to obtain an effect of controlling the absorptance.
An object of the present invention is to provide an optical recording medium arranged to reduce the spot size and raise the linear recording velocity to raise the recording density and transfer rate and enable satisfactory direct overwriting to be performed without deterioration in the durability against repeated use and stable preservation characteristic of the recorded signal.
To achieve the foregoing object, the optical recording medium according to the present invention comprises a recording layer made of at least a phase-change material, wherein a ratio Ac/Aa of absorptance Ac of the recording layer in a state in which the recording layer is crystal with respect to the wavelength of recording/reproducing light and absorptance Aa of the recording layer in a state in which the recording layer is amorphous is 0.9 or higher, and a crystallization enhancing material for enhancing crystallization of the phase-change material is in contact with at least either surface of the recording layer.
The basic idea of the present invention is to employ both absorptance control and enhancement of crystallization. The two components are combined with each other so that the difference between the physical properties of crystal and those of amorphous is reliably compensated. Thus, satisfactory direct overwriting can be performed.
Each of the methods, that is, the absorptance control and the enhancement of crystallization, are able to reduce the foregoing problems to an allowable level. Deterioration in the durability against repeated use and the characteristic of stably preserving a recorded signal can be prevented.
The absorptance control and the enhancement of crystallization are mutually contradictory techniques. If the two techniques are combined with each other, undesirable compensation is expected. Therefore, substantially no attempt of the foregoing combination has been made.
The inventors of the present invention have found that some countermeasure is required to overcome a problem in that the value of jitters cannot be reduced if the absorptance control is performed under a condition of a high linear velocity.
Various investigations carried out by the inventors included combining the absorptance control and the enhancement of the crystallization resulting in the discovery that their advantages can maximally be realized and problems of each techniques can substantially be prevented if the design is properly performed.
The absorptance control is performed such that a contrivance of the lamination structure of the film prevents absorption in the amorphous state of the recording layer. As a result, the erasing sensitivity deteriorates in spite of improvement in the durability against repeated reproduction.
When high speed crystallization is realized by providing the crystallization enhancing layer, the erasing sensitivity can be improved. However, durability against repeated reproduction deteriorates.
When the foregoing techniques are combined with each other, the recording mark can be erased as required i.e., high-speed overwriting is permitted. When erasing of the recording mark is not required, erasing is inhibited. For example, a crosswriting characteristic can be improved and also the preservation stability can be improved. Moreover, a rise in the value of jitters caused from an increase in the number of reproducing operations can be prevented.
The foregoing unexpected facts have been found by the inventors of the present invention.